If you are interested there are great articles concerning ground reaction forces through the feet and bar kinematics during the olympic lifts. Search the NSCA journal for research and you should find a bunch.
Relatively speaking I think you should stay on your heels as long as possible and towards the end of the second pull extend at the ankle and come up on your toes, for a brief moment.
However, I am sure there are people that do it completely different and move big weights…